Method and apparatus for forming an electrical connection

ABSTRACT

A method and apparatus of forming a solder connection between a plurality of elongate bodies, comprises: 
     (i) forming an initial connection between the elongate bodies by inserting them into an induction heatable connecting element of a connector, the connector comprising a dimensionally heat-recoverable sleeve and, retained within the sleeve, the connecting element and a solder insert that is in thermal contact with the connecting element; and 
     (ii) heating the connector (a) by subjecting the connecting element to an alternating magnetic field so that it is heated by induction thereby melting the solder insert, and (b) subjecting the sleeve to hot air and/or infrared radiation, thereby causing the sleeve to recover. 
     The apparatus for applying heat to an elongate connector, comprises a first heat source which comprises an induction coil, and a second heat source arranged to generate hot air or infrared radiation.

This invention relates to the formation of connections between elongatebodies, particularly electrical connections and especially connectionsbetween electrical wires and cables.

In many instances it is desired to form a solder connection between twoor more wires. This can, for example be achieved by means of solderconnection devices comprising a small dimensionally heat-recoverablesleeve which contains a quantity of solder. The wires can be insertedinto the sleeve after the ends have been stripped of insulation, and thedevice can then be heated, for example by means of a hot-air gun or byan infrared lamp, to recover the sleeve about the wires and to melt thesolder inside the sleeve. A device for forming such a solder connectionis disclosed in International Patent application publication No.WO92/00616, the disclosure of which is incorporated herein by reference.That device comprises a metallic connecting element in the form of atapering helical coil of wire located in a dimensionallyheat-recoverably sleeve, and a quantity of solder. The device enables atemporary or initial electrical connection to be formed by screwing thedevice onto the wires and then, for example after the connection hasbeen electrically tested, the device can be heated to form a permanentelectrical connection. By means of such devices it is possible to formvery reliable solder joints which are sealed against ingress ofmoisture. However in many cases a degree of skill is required on thepart of the operator in order to ensure that the solder is fully meltedbut at the same time to prevent overheating of the wire insulation theheat-recoverable sleeve or the like.

Self-regulating induction heating has been used in an attempt to preventoverheating. For example, European patent application, publication No.0371458 discloses a method of terminating an electrical wire at aconnector assembly, in which the connector terminal, comprising a soldertail, has a thin layer of a self-regulating heating source bonded to it.The self-regulating heating source comprises a foil having a first layerof copper or copper alloy which has a low resistance and minimalmagnetic permeability, and a second thin layer of magnetic material suchas nickel-iron alloy. The electrical wire is terminated by placing astripped end of the wire over the solder tail. An alternating magneticfield is then applied to the self-regulating heating source, at afrequency of 13.56 MHz for example, causing the solder tail to heat upand melt the solder and cause the sleeve to shrink. Because the heatingsource is self-regulating, it may be heated to a pre-selected maximumtemperature sufficient to melt the solder and shrink the sleeve.

European patent application No. 0420480 discloses an alternative methodof terminating an electrical wire at a connector assembly, wherein aself-regulating induction heater preform comprising a band of bipartitemetal having a first layer of non-magnetic metal, e.g. copper, and asecond layer of high magnetic permeability metal, e.g. an alloy ofnickel and iron, is crimped around a stripped end of the wire to beterminated. A heat-recoverable sleeve containing a solder preform isthen installed on a connector terminal and the stripped end of the wirewhich has the band of bipartite metal crimped on it is inserted into thesleeve. The bipartite metal band is then heated by induction by placingan inductance coil around the sleeve and applying a high frequencyalternating current, e.g. 13.56 MHz in the coil. The heating of thebipartite metal band causes the solder preform to melt and the sleeve torecover. Optionally, a preliminary assembly step may be carried out,whereby the heat-recoverable sleeve is pre-installed on the connectorterminal by applying a limited amount of heat to a leading end of thesleeve to cause the leading end to recover about part of the terminal.

A further method of using self-regulating induction heating to form asoldered electrical connection is disclosed in European patentapplication, publication No. 0405561. In this method, theself-regulating induction heater comprises a preform that is eitherwrapped around or against a solder preform within a length ofheat-recoverable tubing. The heater preform is formed from a first layerof copper or copper alloy having a thickness of for example 0.05 mm anda second layer of magnetic material such as nickel-iron alloy having athickness of 0.01 mm to 0.015 mm for example. The preform is formed as athin layer and preferably has a spiral shape so that it is easilyreduceable in diameter to permit the sleeve of heat-recoverable tubingto reduce in diameter upon being heated to its recovery temperature.

European patent application, publication No 0371455 discloses adifferent approach to self-regulating induction heating. In thisapproach, a heat-recoverable sleeve containing a solder preform isheated by means of a self-regulating heater strap which is wrappedaround the sleeve. The strap comprises a first layer of copper or copperalloy having a thickness of for example 0.05 mm and a second layer ofmagnetic material such as nickel-iron alloy having a thickness of forexample 0.01 mm to 0.015 mm. The strap is heated either by induction orby direct application of an alternating current, and the heat generatedin the strap melts the solder preform and causes the sleeve to recover.This approach may sometimes be used to seal solder tails of the typedisclosed in EP0371458 when the tails are not used to terminate anelectrical wire. In this case, the strap may be used to heat and shrinkan end region of the sleeve which is not located around the solder tailand the solder tail may be heated by induction in order to heat andshrink the part of the sleeve that is located over the solder tail.

The use of induction, however, as a means of heating solder connectiondevices has a problem in that the degree to which the various componentsof the device are heated depends on the nature of the componentsthemselves as well as the induction heating source. For example thefrequency of the power source that is needed in order to raise theelongate bodies, e.g. copper wires, to the required temperature is notthe same as that needed to melt the solder or to recover the sleeve.This can be seen by considering the skin depth which is given by therelationship ##EQU1## where δ is the skin depth measured in metres, ρ isthe resistivity of the component considered, μ is its relative magneticpermeability and μ is the frequency of the ac field of the work coil.Thus, as the resistivity and magnetic permeability of the variouscomponents differ, the skin depth will differ and will not normallymatch the physical thickness of the components.

According to one aspect of the present invention, there is provided amethod of forming a solder connection between a plurality of elongatebodies, which comprises:

(i) forming an initial connection between the elongate bodies byinserting them into an induction heatable connecting element of aconnector, the connector comprising a dimensionally heat-recoverablesleeve and, retained within the sleeve, the connecting element and asolder insert that is in thermal contact with the connecting element;and

(ii) heating the connector (a) by subjecting the connecting element toan alternating magnetic field so that it is heated by induction, therebymelting the solder insert, and (b) subjecting the sleeve hot air and/orinfrared radiation, thereby causing the sleeve to recover.

According to another aspect of the invention, there is provided anapparatus for applying heat to an elongate connector, comprising

(i) a first heat source comprising an induction coil arranged togenerate an alternating magnetic field, and

(ii) a second heat source arranged to generate hot air or infraredradiation, wherein

(iii) the first source is disposed around a first portion of theconnector, and wherein

(iv) the second source comprises a hollow rigid component that isarranged to surround a second portion of the connector that islongitudinally spaced apart from said first portion of the connector.

The method and apparatus according to the present invention have theadvantage that it is possible for the heat-recoverable sleeve and othercomponents of the connector with greatly differing physical andelectrical properties to be heated by the correct amount duringformation of the connection. A problem associated with previous methodsand apparatus for forming solder connections is that one or more of thedifferent components of the connector are normally overheated in orderto ensure that another of the components is heated sufficiently. Forexample, if only an external source of heating, e.g. hot air or infraredradiation, is used both to recover the heat-recoverable sleeve and tomelt the solder, the recoverable sleeve will overheated because all ofthe heat required to melt the solder needs to pass through the sleeve.Overheating may, for example, degrade the properties of the sleeve andis in any case inefficient and time-consuming. Alternatively, however,if only an internal source of heating, e.g. by induction, is used thesolder may be overheated because the thermal conduction from an internalheating element to the solder is much more rapid than conduction fromthe heating element to the extremities of the heat-recoverable sleeve orfrom the heating element along the elongate bodies, e.g. wires andthrough the wire insulation, to the extremities of the heat-recoverablesleeve. Overheating of the solder may cause the solder to `wick` alongthe wires or `squirt` out of the connector, thereby causing shortcircuits or `dry` connections. In addition, overheating of the soldermay cause overheating of the sleeve in the vicinity of the solder, andin any case is inefficient and time-consuming.

The present invention solves, or at least alleviates, the above problemsassociated with previous methods and apparatus, since it normallyenables the correct amount of heat to be supplied to the solder in orderto melt it and the correct amount of heat to be supplied to the sleevein order to cause it to recover, substantially without overheating anycomponent of the connector or, for example, the insulation of wiresconnected by means of the connector.

According to a preferred embodiment of the invention, the connector isheated by both the hot air and/or infrared radiation substantiallysimultaneously. This has an advantage in that not only is overheating ofcomponents of the connector normally avoided, but also the time taken tomelt the solder and recover the sleeve can normally be reducedsignificantly in comparison to conventional methods, due to the twosources of heat complementing each other.

The present invention is especially advantageous for forming a solderconnection by means of a connector which has part of its dimensionallyheat-recoverable sleeve extending beyond at least one end of theconnecting element. In this case, heating the connector solely byinduction can be inefficient and time-consuming since the further thesleeve extends away from the connecting element, the less efficient andmore time-consuming is the transfer of heat from the connecting elementto the end of the sleeve. Hence, according to a preferred aspect of theinvention, the hot air and/or infrared radiation is applied to a portionof the sleeve which is longitudinally spaced apart from a portion of thesleeve which retains the connecting element, which is heated byinduction.

The connecting element of the connector which is used to form the solderconnection may be formed from substantially non-magnetic material, forexample copper and particularly hard temper copper. Preferably, however,the connecting element is formed from high magnetic permeabilitymaterial. The phrase "high magnetic permeability material" is intendedto mean a material having a relative magnetic permeability, at low Hfields, of at least 5, more preferably at least 10 and especially atleast 100, but will often be 1000 or more. The connecting element isnormally hollow and open-ended so that the ends of the bodies can beinserted therein, and preferably has a screw-threaded interior so thatthey can be screwed into it and will then be temporarily held therein.The connecting element may be made in a number of ways and from a numberof materials. The heat may be generated in the element by hysteresislosses or by eddy current losses or by both mechanisms depending on thematerial from which the element is formed. For example the element maybe formed from a conductive, substantially non-magnetic material such ascopper, or a ferromagnetic material such as low carbon steel, in whichcase the heating effect will be caused by eddy current losses, or it maybe formed from a ferrimagnetic material such as a ferrite in which casethe heating effect will be due to hysteresis losses.

The connecting element can be made from a wire by coiling it up,normally into a frusto-conical configuration so that the wire itselfprovides the screw thread on the interior of the element. In this casethe wire forming the element may be provided with a pair of flat facesextending along its length that join to form a ridge, for example it mayhave a polygonal cross-section, to make the screw thread morepronounced. Such an element would have a form generally as shown ininternational patent application No. WO/9200616. This form of element,as can others, may be formed from materials such as copper or steel,especially low carbon steel, or from ferritic stainless steel.Alternatively, the element may be formed from a solid block, for examplea machined block or formed by other methods, in which case it may beformed from a metal as described above or from a non-metallic highpermeability material such as a sintered ferrite, especially one havinga Curie temperature in the range of from 225° to 250° C. Such a materialhas the advantage that it enables the heating method to heat the articleto a temperature in the region of the Curie temperature, so causing thesolder to melt (eg. an Sn₆₃ Pb₃₇ eutectic will have a melting point of183° C.) but the heating efficiency will fall off rapidly attemperatures above the Curie point of the element and thereby limit thetemperature rise of the article to one governed by the Curie point ofthe element. If it is desired to improve the degree of control over theheating step, it is often possible to monitor the reduction of themagnetic field strength in the region of the connecting element as theelement passes through its Curie temperature and to use this reductionto control the termination of the heating step, eg. by stopping power tothe heating coil.

According to the invention the recoverable sleeve will recover, and anysealant will fuse, principally due to the effect of the hot air and/orinfrared radiation, whereas the copper conductors to be connected willbe heated almost entirely by thermal conduction from the connectingelement. In most instances the solder will be heated principally bythermal conduction from the connecting element although a significantamount of heating of the solder may occur due to the hot air or infraredheater. Where the connecting element is in the form of a coil, thesolder will flow through the windings of the coil into its interior andso connect the conductors with the element, and if the element is formedfrom a solid block of material, it will be necessary to form a number ofholes in the element to allow the solder access to the interior of theelement.

As stated above the sleeve is dimensionally heat-recoverable, that is tosay the article has a dimensional configuration that may be madesubstantially to change when subjected to heat treatment. Usually thesearticles recover, on heating, towards an original shape from which theyhave previously been deformed but the term "heat-recoverable", as usedherein, also includes an article which, on heating, adopts a newconfiguration, even if it has not been previously deformed.

In their most common form, such articles comprise a heat-shrinkablesleeve made from a polymeric material exhibiting the property of elasticor plastic memory as described, for example, in U.S. Pat. Nos.2,027,962; 3,086,242 and 3,597,372. As is made clear in, for example,U.S. Pat. No. 2,027,962, the original dimensionally heat-stable form maybe a transient form in a continuous process in which, for example, anextruded tube is expanded, whilst hot, to a dimensionally heat-unstableform but, in other applications, a preformed dimensionally heat-stablearticle is deformed to a dimensionally heat-unstable form in a separatestate.

In the production of heat-recoverable articles, the polymeric materialmay be cross-linked at any stage in the production of the article thatwill enhance the desired dimensional recoverability. One manner ofproducing a heat-recoverable article comprises shaping the polymericmaterial into the desired heat-stable form, subsequently cross-linkingthe polymeric material, heating the article to a temperature above thecrystalline melting point or, for amorphous materials the softeningpoint, as the case may be, of the polymer, deforming the article andcooling the article whilst in the deformed state so that the deformedstate of the article is retained. In use, since the deformed state ofthe article is heat-unstable, application of heat will cause the articleto assume its original heat-stable shape.

Any material to which the property of dimensional recoverability may beimparted may be used to form the sleeve. Preferred materials includelow, medium or high density polyethylene, ethylene copolymers, eg. withalpha olefins such as 1-butene or 1-hexene, or vinyl acetate, polyamidesor fluoropolymers, eg. polytetrafluoroethylene, polyvinylidine fluorideor ethylene-tetrafluoroethylene copolymer.

The solder employed in the connector is a soft solder as distinct frombrazing material. The solder may, for example, simply be in the form ofan Sn₆₃ Pb₃₇ eutectic composition which will melt as the device isheated and the sleeve recovers, or more than one solder compositionhaving differing melting points may be employed, as described inInternational Application No. WO88/09068. In this form of device,melting of the higher melting point component, eg. Sn₉₆.5 A_(g3).5eutectic will provide a visual indication that the device has beenheated sufficiently to melt the lower melting point composition and toform a satisfactory solder joint. If desired the lower melting pointsolder may be a non-eutectic composition and, for example as describedin International Application No. WO90/09255, the higher and lowermelting point solder compositions may together form a eutecticcomposition. For example, a non-eutectic Sn₆₀ Pb₄₀ lower melting pointcomponent may be employed with a higher melting point component formedfrom pure tin in relative amounts that an Sn₆₃ Pb₃₇ eutectic is formed.The disclosures of these two patent applications are incorporated hereinby reference. An advantage of employing a two component solder, andespecially a tin, Sn₆₀ Pb₄₀ combination is that it reduces thepossibility of "wicking" that is to say, travel of the solder along theconductors and away from the joint area due to capillary action by thestranded conductors, which can be caused by prolonged heating of thedevice.

The solder may be positioned anywhere where it will be able to flow intothe connecting element to form a solder joint and where it is in goodthermal contact with the element. The solder may be employed in the formof a ring or in any other form for example a ball, and may be disposedsymmetrically about the sleeve axis or offset from it. The solderelement may, for instance, be located at the smaller diameter end of afrusto-conical connecting element in which case it may be in the form ofa ball or plug, or it may be located in the region of a large diameterend of the connecting element, for example in the form of a ring.Preferably the solder is in the from an element that surrounds theconnecting element, especially where the connecting element is in theform of a coil so that the fused solder can flow through the windings ofthe coil to the interior thereof. More than one quantity of solder maybe employed, for example where the connecting element has more than onetapering internal surface for forming a splice.

The hot air and/or infrared heating step may be carried out before orafter the induction heating step or simultaneously therewith. If the twoheating steps are carried out simultaneously the hot air gun or infraredlamp may be incorporated into the induction heating coil.

The infrared heating source may be provided by a hollow rigid componentwhich can be excited by an induction coil if chosen of suitablematerial. It may be convenient to use a single source of inductionheating by combining the induction coil of the infrared heating sourcewith a coil that is used to heat the connecting element. In thisarrangement the entire heating of the connector and the connection maybe carried out substantially simultaneously.

As mentioned above, the second heat source of the apparatus according tothe invention comprises a hollow rigid component. According to apreferred embodiment of the apparatus according to the invention, thesecond heat source is arranged to be heated by induction, and onceheated, to generate infrared radiation. It is particularly preferredthat the second heat source is arranged to be heated by the first heatsource. This has the advantage that only one source of power is neededto heat an article both by induction and by infrared radiation.

Depending on the particular requirements and the composition of theconnector, the hollow rigid component may be formed from any of avariety of different materials and may have any of a number of differentforms. For example, for certain applications the component may be formedfrom a material of high magnetic permeability, e.g. a ferrite or lowcarbon steel, but for other applications, the element may be formed fromsubstantially non-magnetic material, e.g. copper. The choice of materialwhich best suits the particular requirements will normally be made on atrial and error basis. Also depending on the particular requirements,the component may, for example, have a substantially cylindrical orconical shape, or it may comprise at least one coil.

In addition to the method and apparatus, the present invention alsoprovides a solder connection between a plurality of elongate bodies thathas been formed by the method according to the invention.

The method and apparatus according to the present invention will now bedescribed by way of example with reference to the accompanying drawingsin which:

FIG. 1 is a side sectional elevation of a connector that is employed inthe present invention;

FIG. 2 is a side section view of the connector of FIG. 1 together withwires during the heating step;

FIG. 3 is a side sectional elevation of a second form of connector;

FIGS. 4 and 5 are partially cut-away views of alternative forms ofconnecting element; and

FIGS. 6 and 7 are schematic representations of one form of apparatusaccording to the invention, showing a connector being inserted into theapparatus and heated.

Referring to the accompanying drawings, FIG. 1 shows a connector forforming a solder joint between a number of electrical wires 2 whichcomprises a dimensionally heat-recoverable sleeve 3 formed fromcrosslinked and expanded polyvinylidine fluoride, and a connectingelement 4 formed as a frusto-conical spring or coil of low carbon steelwire. The steel wire can have a cross-section for example in the form ofa square or a rhombus in which sides, forming faces on the wire, arearranged at an angle of approximately 60° to one adjacent side and at anangle of approximately 120° to the other adjacent side. The wire iscoiled up so that the ridges formed by the faces that are at 60° to eachother are located on the interior and the exterior of the element, theinterior ridge forming a screw thread for holding the wires to beconnected. One end of the wire located at the smaller diameter end ofthe connecting element 4 is bent so that it extends across the axis ofthe coil and prevents over insertion of the conductors to be connected.In some instances it may be advantageous to expand the diameter of thecoil 4 by opening out the ends of the copper wire 5 and retaining themin their new position.

A ring 8 of Sn₆₃ Pb₃₇ eutectic solder is located about the externalsurface of the connecting element 4 between the connecting element andthe heat-shrinkable sleeve 3. As shown, the solder ring is relativelythick and short, its axial length being only approximately twice itsradial thickness, although in many instances it may be desirable for thering to be thinner and longer in order to improve the thermal contactwith the connecting element.

One end of the sleeve in the region of the smaller diameter end of theconnecting element is pre-recovered onto a spherical sealing element 10formed from a fusible polymeric material, eg. polyethylene, and afurther sealing element 11 in the form of a ring is located within thesleeve adjacent to the other end of the connecting element 4.

In order to form an electrical connection between the wires 2 in abundle, their ends are stripped of insulation and inserted into the openend of the connector 1 until they abut the end of the end of the wire 5that has been bent across the axis of the coil and acts as a stop. Theconnector 1 is then given a small twist to screw the wires 2 into theconnecting element 4 and hold the connector on the wires. The wires andconnector are then both inserted into an induction heating coil 12 whichis powered up. During this process the connecting element heats up andcauses the solder ring 8 to melt and flow through the windings of thecoil to its interior and so form a solder bond between the wires and theconnecting element.

Simultaneously with the induction heating step, the device is brieflyheated externally with hot air by means of a hot air gun 13. Thetemperature, flow rate and heating cycle time of the hot air gun is setso that the hot air will not, on its own, melt the solder ring 8, but itwill cause the heat-recoverable sleeve 3 to shrink about the wires andthe sealing ring 11 to melt. A stub splice that is sealed againstmoisture ingress is thereby formed.

FIG. 3 shows a form of connector according to the invention of the formdescribed in International patent application No. PCT/GB92/02257 forconnecting one or more ground leads to the shield of a coaxial cable.This form of connector comprises a heat-recoverable polyvinylidinefluoride sleeve 31 that contains a generally diabolo shaped connectingelement 32 wraps of fluxed Sn₆₃ Pb₃₇ eutectic solder 33' and 33", and apair of fusible polyethylene sealing rings 34' and 34", one sealing ringbeing located at each end of the connecting element 32. As describedabove, the connecting element has been formed from by coiling a lowcarbon steel wire that has a square cross-section.

In use a central portion of the outer jacket 35 of a coaxial cable 36 isremoved in order to expose a portion of the braid 37 forming the screen.One or more ground leads 38 can be inserted into one open end of theconnecting element 32 and the element 32 can then be twisted about thecoaxial cable 36 and the ground lead in order to grip the ground lead.The connector can be heated by means of an induction coil and hot-airgun as described in FIGS. 1 and 2 to form a sealed splice.

The connecting element 32 is capable of expanding at its waist ifnecessary in order to fit over coaxial cables of a range of diameters,the maximum diameter being determined by the size of the chamber formedby the central section 38 of heat-recoverable sleeve 31. Provision ofthe solder 33 in the form of wrap will allow the solder to accommodateany increase in size of the connecting element.

FIGS. 4 and 5 show two further connecting elements and solder rings thatmay be employed in connectors used in the present invention. This formof element 40, frusto-conical as shown in FIG. 4 and diabolo as shown inFIG. 5 are formed from a sintered ferrite, eg. a Mn Ni or Ni Zn ferritehaving a Curie point between 200° and 250° C. The elements 40 are formedby moulding or machining solid bodies of the ferrite. Usually it will benecessary for holes 41 to be provided in the elements in order to enablethe solder 42 to flow into the interior of the element after fusing. Theelements 40 may be provided with teeth or a screw thread 43 on theirinterior surface in order to allow the elements to grip the strippedwire ends that are to be connected by a simple twisting action asdescribed above. These forms of connecting elements may be employed inconnectors as shown in FIGS. 1 and 3 exactly as described above with theexception that the rate at which the elements 40 will generate heat willfall considerably as the element passes through its Curie temperature,so that the risk of overheating in the induction heating step isreduced.

FIG. 6 is a schematic representation of the connector 1 prior to beinginserted into a hollow rigid component 52 and induction coils 51' and51" of heating apparatus 50. The induction coils 51' and 51" maycomprise separate coils or they may be parts of a single coil. Thecomponent 52 comprises a substantially cylindrical component locatedinside the induction coil 51'.

FIG. 7 shows the connector 1 disposed in the apparatus 50 and after theconnection has been made. To achieve this an alternating current hasbeen passed through the induction coils 51' and 51", which has generatedan alternating magnetic field inside the coil. The alternating magneticfield heated the connecting element 4 in a first heating zone A byinduction in the coil 51", and thermal conduction from the connectingelement has melted a solder ring 8 (shown in FIG. 1). The alternatingmagnetic field generated by the coil 51' has heated the hollow rigidcomponent 52 by induction in a second heating zone B, which has causedthe component to radiate infrared radiation, thereby heating theheat-recoverable sleeve 3 in the zone B and causing it to recover aboutthe wires 2.

We claim:
 1. A method of forming a solder connection between a plurality of elongate bodies, which comprises:(i) forming an initial connection between the elongate bodies by inserting them into an induction heatable connecting element of a connector, the connector comprising a dimensionally heat-recoverable sleeve and, retained within the sleeve, the connecting element and a solder insert that is in thermal contact with the connecting element; and (ii) heating the connector (a) by subjecting the connecting element to an alternating magnetic field so that it is heated by induction thereby melting the solder insert, and (b) simultaneously subjecting the sleeve to at least one of hot air and infrared radiation, thereby causing the sleeve to recover.
 2. A method as claimed in claim 1, wherein the hot air and/or infrared radiation is applied to a portion of the sleeve which is longitudinally spaced apart from a portion of the sleeve which retains the connecting element.
 3. A method as claimed in claim 1, wherein the infrared radiation is applied to the sleeve by means of a heating element located outside the sleeve, which element is heated by induction.
 4. A method as claimed in claim 3, wherein the alternating magnetic field that heats the connecting element is produced by an induction coil which also heats the heating element.
 5. A method as claimed in claim 1, wherein the connecting element has an internal screw thread and the initial connection between the elongate bodies is formed by screwing the bodies into the connecting element so that they are held therein.
 6. A method of forming a solder connection between a plurality of elongate bodies, which comprise:(i) forming an initial connection between the elongate bodies by inserting them into an induction heatable connecting element of a connector, the connector comprising a dimensionally heat-recoverable polymeric sleeve and, retained within a longitudinal portion only of the sleeve, the connecting element and a solder insert that is in thermal contact with the connecting element; and (ii) heating the connector (a) by subjecting said longitudinal portion of the sleeve enclosing the connecting element to an alternating magnetic field so that the connecting element is heated by induction thereby melting the solder insert, and (b) simultaneously subjecting the sleeve beyond said longitudinal portion to at least one of hot air and infrared radiation, thereby causing the sleeve to recover.
 7. An apparatus for applying heat to an elongate connector, comprising:(i) a first heat source comprising an induction coil arranged to generate an alternating magnetic field, and (ii) a second heat source arranged to generate infrared radiation, wherein (iii) the first source is disposed around a first portion of the connector that encompasses an induction heatable connecting element, and wherein (iv) the second source comprises a hollow rigid component that is arranged to surround a second portion of the connector that encompasses a dimensionally heat-recoverable polymeric sleeve and that is longitudinally spaced apart from said first portion of the connector.
 8. An apparatus as claimed in claim 7, wherein the second heat source is arranged to be heated by the first heat source. 